1. Field of the Invention
The invention relates to the processes practiced by and construction of wastewater treatment plants. More specifically, the invention relates to a system for removing solids from combined sewer overflows.
2. Background Art
Combined sewer overflows (CSOs) are discharges of wastewater from a collection system that carries sanitary and storm water. Combined sewer systems deliver the wastewater to a wastewater treatment plant (WWTP). However, melting snow or heavy, sustained rain may result in the capacity of conventional storage and ducting systems to be exceeded. Accordingly, access wastewater sometimes charge directly into surface water. In the past, this practice has been accepted in situations where it was believed that receiving waters could handle the overflow without serious damage. But, it would clearly be undesirable if sewers were to backup into streets and basements.
Combined sewer systems are frequently found in older urban communities, in which discharges from CSOs may significantly degrade water quality. It is known that CSO discharges contain pollutants found in storm water runoff as well as “sanitary” wastes. For example, storm water may transport pollutants deposited on the ground from air pollution, and sub-optimal home and business maintenance practices. Typical CSO discharges have the same pollutants as sanitary wastewater: for example, phosphorus, nitrogen, metals and bacteria, suspended solids, and other pollutants associated with storm water.
One function of a retention treatment basin is to eliminate the flow of raw sewage discharge into a receiving environment. These facilities, for example, may have a basin that is sized to provide a 30 minute retention of CSOs generated by a one year, one hour storm event. The treatment process for combined overflow involves retaining the overflow before it can reach a drain field. CSOs are designed to handle or bring sewage effluent to a treatment plant during dry weather. During wet weather infrastructure is provided to also carry storm water that enters the system. This storm water deluges the system from time to time. It does not allow pipes with an inherent-capacity constraint to deliver all effluent to the treatment plant. Further, treatment plants may not be large enough to deal with an incoming volume over a lengthy period. In such situations, effluent may overflow certain points in the system.
Accordingly, basins have been designed to retain excess effluent before it goes to a drain field. If a deluge is relatively limited in intensity or duration, a basin may retain the entire flow. The effluent receives settling, disinfection, and skimming as treatment steps before subsequent flow to the receiving water. In this way, primary treatment and disinfection produce a treated effluent that is diluted by rainwater. As a result, the treated effluent is of reasonable quality—not as high as can be produced by a treatment plant—but better than the quality of effluent reaching a receiving water under historical practices.
In some conventional retention treatment basins, a screening facility is provided. That facility arrests the flow of effluent solid material. Typically, the screened material must be mechanically raked and then lifted to ground level to be trucked out. However, such practices may violate construction restrictions that may be environmentally imposed.
Accordingly, it would be desirable to have an innovative screening system that is totally below ground and does not require mechanical raking.
Against this background, it would be desirable to have a system that works in most cases on a gravity system, rather than a pumping system with its engineering and operational consequences, such as backing up, flooding, and other adverse consequences.
Further background information is available in “COMBINED SEWER OVERFLOW TECHNOLOGY FACT SHEET,” EPA—832-F-99-042 (September 1999). This fact sheet is incorporated herein by reference.
Among the art revealed by a preliminary search that preceded the filing of this application were the following U.S. references: U.S. Pat. Nos. 5,034,122; 5,290,454; 5,435,664; 5,597,477; 5,658,458; 5,672,281; 5,824,222; 5,910,249; 6,419,094; 6,475,397; 6,773,606; 6,70,351; 2002/0148781; 2002/0148782; 2002/0153305; 2002/0153306; 2003/0010713; 2003/0085182; 2003/0085310; and 2003/0089088.
Primary concerns relating to retention treatment basins in conventional CSOs are their costs and environmental impact.